A system has previously been proposed in which rain detectors or dirt detectors are used. These are detectors which sense the degree of transparency of the windshield: they take the form which is illustrated diagrammatically in FIG. 1 of the accompanying drawings, and will be best explained by reference to FIG. 1.
With reference to FIG. 1, therefore, a point light source 10, supplied through an emission circuit 12, illuminates the windshield 14 from the inner side of the latter. The light is transmitted into the windshield through an optical interface 16, which may be prismatic or lenticular, at an angle of incidence such that the light penetrates into the glass of the windshield and is then subjected to multiple total reflections within the thickness of the glass. An optical interface 18 is placed at a distance corresponding to a whole number of reflections of the light beam (FIG. 1 shows four such reflections). The optical interface 18, like the interface 16, may be prismatic or lenticular, and enables the beam to pass out of the windshield and to be directed towards a light sensor 20. This light sensor is coupled to a detection circuit 22, which processes the signal received from the sensor 20 and is arranged to transmit a starting signal to a power control circuit 24. The latter is the control circuit for a motor 26 which drives the windshield wipers of the system. The power control circuit is of course also arranged to be started or stopped by a manual control switch 28 which takes priority over the circuit 22.
When the windshield is clean and dry, the light from the source 10 is subject to multiple total internal reflection, with a very low attenuation taking place within the windshield. However, when it is wet or covered with snow or dirty, this light is absorbed or diffused at those points where perfect total reflection back towards the interior of the windshield ought to be taking place. In consequence, the light sensor 20 receives a reduced intensity of light as compared with that which it receives when the windshield is fully reflective. It is this difference in light intensity which is utilised by the detection circuit 22 in order to determine whether or not it is appropriate to commence a wiping operation.
One of the disadvantages of rain detectors as proposed up to the present time is the fact that they perform a detection of rain in a very limited linear zone of the windshield; the process is thus too coarse to identify nuances in the condition of the windshield. In this connection, it will be understood with reference to FIG. 1 that the rain detector will only supply a rain detection signal if the rain drops fall exactly on the points on the outer face of the windshield at which internal total reflection of the light ought to take place. For example, in FIG. 1 there is shown one rain drop 30 which has fallen on one of these points, while other rain drops 32 and 34 have fallen on other points where they have no influence at all on the detector. In addition, with this detector there is no way of discriminating between the natures of different effects which cause a variation in the intensity of incident light to take place, so that the detection circuit 22 is supplied with very little information on which to base a decision as to whether to leave the system in its active or inactive state, or alternatively whether for example to commence a wiping operation at low speed or high speed or intermittently, or to activate the windshield washer with three sweeps of the windshield wipers, and so on.